1. Field of the Invention
The present invention relates to a cathode ray tube having a cancel coil for an external magnetic field such as earth magnetism or the like.
2. Description of the Prior Art
As shown in FIG. 1 of the accompanying drawings, a color cathode ray tube comprises a tube envelope body 1 of a CRT (cathode ray tube) assembly, a phosphor screen 2 formed on the inner surface of a panel of the tube envelope 1, a color selecting electrode (e.g., aperture grill, shadow mask, etc.) 3 placed opposite the phosphor screen 2 and an electron gun 4 disposed in a necked-down portion of the tube envelope 1. An electron beam 5 emitted from the electron gun 4 is deflected by a deflection yoke 6 at a predetermined period in the horizontal and vertical directions so as to irradiate the phosphor screen 2, whereby the phosphor screen 2 is rendered luminous.
The electron beam 5 deflected by the deflection yoke 6 is affected by various forces of a magnetic field that was formed within the tube envelope 1 by earth magnetism before reaching the phosphor screen 2 so that the electron beam 5 is deviated from its path.
When a color cathode ray tube, for example, is directed in the north and south direction in use, the electron beam 5 is deviated from its path by a horizontal component (in the vertical direction to the picture screen) of earth magnetism in the north and south direction. If the amount that the electron beam 5 is deviated from its path is large, then such amount appears as a phenomenon in which the entirety of a picture screen 10 is rotated. This phenomenon will hereinafter be referred to as an image rotation for simplicity. If such image rotation becomes 0.5 or more at a center angle .theta., then such image rotation becomes a nuisance for the viewer.
In order to suppress the influence of the horizontal earth magnetism component, according to the prior art, external peripheral portions of the panel and the funnel portion of the cathode ray tube are surrounded by an external magnetic shield member to magnetically shield the cathode ray tube from the horizontal earth magnetism component or an annular internal magnetic shield member is provided within the tube envelope 1 to magnetically shield the cathode ray tube from the horizontal earth magnetism component.
Further, a degauss coil is provided at one portion outside the tube envelope to degauss magnetic members disposed in and out of the tube envelope.
Present color cathode ray tubes tend to dispense with the external magnetic shield member because the color cathode ray tubes are made large in size but are relatively short.
The internal magnetic shield member, however, is poor in shield characteristic against the north and south horizontal earth magnetism component which considerably contributes to the rotation of the picture as compared with the external magnetic shield member. Therefore, the magnetic shield based on the degauss coil and the internal magnetic shield member has a limit as a countermeasure for preventing the picture from rotating.
On the other hand, as the color cathode ray tube becomes large in size, such as a high definition television cathode ray tube or the like, the amount of image rotation is increased and becomes conspicuous.